The present invention generally relates to automotive mirrors for vehicles and to vehicle navigation. More specifically, the present invention relates to combining rearview mirror assemblies and positioning systems.
Vehicle positioning systems are known and commonly used in vehicles for purposes relating to vehicle navigation and tracking systems. Currently, two such positioning systems are the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS). Both systems utilize a constellation of satellites that transmit microwave signals toward the earth that are received by a ground-based microwave receiver and used to determine the position of the receiver on the earth""s surface. Such systems are capable of a high degree of accuracy. Therefore, a great deal of research has been conducted to construct navigation systems that may be readily incorporated into a vehicle.
Positioning systems have also been used in vehicles with respect to communication systems, particularly emergency communication systems, whereby a vehicle occupant making an emergency call using a cellular telephone need not actually know the vehicle""s exact location in order to have emergency vehicles dispatched to that location. An example of such a system is the ONSTAR(copyright) system from General Motors Corporation. Other uses of positioning systems in vehicles include identifying the time zone that the vehicle is currently in, and determining which zone of magnetic variance the vehicle is in for purposes of calibrating an invehicle electronic compass. See U.S. Pat. Nos. 5,724,316 and 5,761,094, respectively.
Despite all the research that has been conducted and all the literature that has been generated relating to the use of positioning systems in vehicular applications, there continue to be problems associated with using GPS and GLONASS type systems (i.e., satellite positioning systems) under certain conditions. A particular concern is the unavailability of GPS/GLONASS signals in parking structures, in large cities with tall buildings (i.e., xe2x80x9curban canyonsxe2x80x9d), and similar GPS/GLONASS hostile environments. GPS/GLONASS satellites transmit signals that are very low power and high frequency (e.g., 1.5 gigahertz). Consequently, reception of GPS/GLONASS signals are typically limited to line of sight and are easily blocked by most structures that block the line of sight between the satellite and the receiver. This problem is noticeable in urban canyons where tall buildings or skyscrapers block the direct line of sight. The problem is also apparent in parking structures, residential garages, any covered or indoor facility, and if the vehicle attitude restricts the line of sight between the antenna and the satellites (e.g., if the vehicle crashes and is inverted). The problem may also occur due to natural barriers such as canyons and foliage. The signal may even be attenuated by seemingly transparent objects such as low-E glass windows in a vehicle. Further, GPS/GLONASS positioning systems typically require signals from at least four satellites in order to make accurate position calculations. Therefore, accurate GPS/GLONASS location information may be disrupted even when signals from three GPS/GLONASS satellites are available.
The prior art has attempted to minimize this problem by using the technique of dead reckoning to provide location information when GPS/GLONASS data is interrupted. Dead reckoning has been used for many years, notably by pilots of aircraft and operators of watercraft. Using this technique, a person starts from a known location and carefully observes the direction and distance of travel to estimate the current location of the person or vessel. Vehicle based dead reckoning systems operate on this same principle and use several sensors to automatically make the position estimates. Some of the sensors used by dead reckoning systems include gyroscopes, ABS (antilock braking system), accelerometers, odometers, and other vehicle sensors. There are many different methods for generating dead reckoning estimates.
Some methods use a gyroscope that provides heading change information that may be combined with odometer information to generate a reasonable estimate of location. However, gyroscopes are expensive and become inaccurate over time. Other methods use the vehicle""s ABS system to provide information on the rotation of each wheel that may be translated into distance traveled and heading changes. Using a vehicle""s ABS, a heading change is estimated from differences in wheel rotation. For example, when a vehicle makes a left turn, the left wheel turns fewer revolutions than the right wheel. While these systems work reasonably well over short distances and for brief periods, they continue to be deficient in many regards.
First, dead reckoning systems are only estimates of vehicle location and must be frequently updated with actual location information from a GPS or similar positioning system. Secondly, they can be confused by turning the vehicle off, making many tight turns, backing up, being towed, and the like. Third, they may be expensive or difficult to implement if they require, for example, an expensive gyroscope, or if they require data from remote vehicle systems such as an ABS system. Finally, systems requiring access to various vehicle systems complicate installation and are not well suited for retrofit installations.
In addition, there are problems with GPS/GLONASS systems even when satellite signals are available and functioning properly. Under normal conditions, the GPS signals provide accuracy to within about 60 feet. This may be improved to some extent using various complex techniques such as correlators and the like. However, the error remains significant.
Therefore, there exists a need for a system and method to reduce problems associated with vehicle based GPS/GLONASS systems, provide accurate location data in urban canyons and in parking structures, and improve the accuracy and availability of GPS/GLONASS data.
Accordingly, it is an aspect of the present invention to solve the above problems by providing a rearview mirror assembly incorporating components of a Loran positioning system. Loran is an acronym for long range navigation and is a navigation system that has been in existence since the 1970""s. It comprises a network of land based transmitters broadcasting time sensitive signals that a receiver can translate into position estimates.
Another aspect of the invention is to improve the accuracy of position estimates. Yet, another aspect of the invention is to reduce the need for dead reckoning systems. Still another aspect of the invention is to reduce costs by integrating Loran components into a rearview mirror assembly. Another aspect of the invention is to provide a navigation system that is easily retrofit into vehicles.
To achieve these and other aspects of the invention, an inside rearview mirror assembly constructed in accordance with the present invention comprises a mirror housing for mounting a reflective member, a mounting foot for securing the assembly proximate to the front windshield of a vehicle, and at least one component of a Loran positioning system mounted on the mirror assembly. In one aspect of the invention the Loran positioning system replaces the GPS/GLONASS systems. In another aspect of the invention both Loran and GPS/GLONASS positioning systems are used together.
The accuracy of position estimates is improved in a number of ways. First, using Loran alone, position estimates within about 40 feet are possible. Further, proposed upgrades to the Loran transmitters and receivers may improve this accuracy in the near future. Second, accuracy may be improved by combining Loran systems with GPS/GLONASS systems. This may be accomplished in several ways. One method is to average or otherwise combine the position estimates from both the Loran system and the GPS/GLONASS system. If both systems are deemed accurate, then the location estimates can be merely averaged. However, if one system is deemed more accurate that the other, the location estimates can be weighted accordingly. In the alternative, the less accurate system can be used to detect a failure condition in the more accurate system and to operate as a backup for the primary system.
Other methods to improve accuracy include the Loran system receiving differential GPS/GLONASS corrections or the Loran transmitters operating as pseudolites for the GPS/GLONASS system.
Another aspect of the Loran positioning system is that it operates in GPS/GLONASS hostile environments and therefore provides improved location estimates over GPS/GLONASS systems and dead reckoning systems. Therefore, Loran broadcasts can be received inside of parking structures, in urban canyons, and inside of buildings. Therefore, Loran may operate as a replacement for dead reckoning systems.
An advantage of the Loran system is that all the major components may be packaged into the rearview mirror assembly thereby reducing manufacturing costs and simplifying installation.
Yet another advantage of the Loran system is that Loran signals may be received through low-E glass which is often used in vehicles. The low-E glass coatings attenuate the 1.5 gigahertz signals of the satellite navigation systems. Therefore, the coating must be masked in the area around the GPS antenna. Loran signals are high power and low frequency and are not significantly attenuated by low-E glass coatings. Therefore, no masking is required.
Still yet another advantage of the Loran system is that the Loran antenna can receive Loran signal regardless of vehicle attitude. Therefore, even if a vehicle becomes inverted as a result of an accident, a Loran based system may still be able to provide vehicle location.